Write a note on $S_{N}1$ reaction.

(N/A) The reaction in which the rate is independent of the concentration of the nucleophile and depends only on the concentration of the substrate is called an $S_{N}1$ reaction. Thus,an $S_{N}1$ reaction follows first-order kinetics.
$(CH_{3})_{3}C-Br + OH^{-} \rightarrow (CH_{3})_{3}C-OH + Br^{-}$
Rate $= k[(CH_{3})_{3}C-Br]$
Mechanism:
Step-$I$: The polarized $C-Br$ bond undergoes cleavage slowly to form a carbocation and a bromide ion. This is the rate-determining step.
$(CH_{3})_{3}C-Br \xrightarrow{\text{slow}} (CH_{3})_{3}C^{+} + Br^{-}$
Step-$II$: The carbocation thus formed is then attacked by the nucleophile rapidly to complete the substitution reaction.
$(CH_{3})_{3}C^{+} + OH^{-} \xrightarrow{\text{fast}} (CH_{3})_{3}C-OH$